I think the problem you are having is because you can't mix hardware when mining. You can run multiples of the same card, but not two completely different cards. Sometimes even the same model card by different manufacturers is problematic.
Even if you have different miners for the different cards? GUIMiner didn't even start. When no compatible cards (or drivers) are installed, at least I get an error message. I could run it and mine with the 4850 and a GeForce GTS 240 simultaneously. I ran two miners (tabs) in GUIMiner, it worked perfectly.